Login Crash after newest 17.10 update

Updating this morning my (usually daily updated) ubuntu 17.10 beta (the badly needed download of fixed Thunderbird took a bit of time). While waiting - updater still running - I changed a setting in System Monitor/System Load indicator: changed width from 100 to 400 pixels. Then restarting the System Monitor Indicator app - system update was still running. Maybe a coincidence, but as soon as the System Monitor Indicator opened, the whole session abruptly closed, I landed at login. I tried logging in as usual through Gnome/Wayland (default) but after turning purple it is jumping back to login screen. So i tried x-Org login. My desktop background image shows for 1 second, then back to login as well. What can I do? I really need some data from the machine and actually would just like to continue using and life-testing. Please help, thanks!

You might try selecting an older kernel version from the grub menu.
Or you could try logging in to a virtual terminal (ctrl-alt-F1) to copy your data to an external storage medium.
Or you could use a different machine for a remote login (e.g. ssh).
Or you boot an Ubuntu installer from DVD or bootable USB stick in the "Try Ubuntu without installing" mode, mount the hard disk and access your data.